If a person starts his Ramadan while in Saudi Arabia because he was there when it started, it means he has found Ramadan two days earlier than in Pakistan for example, where he arrived later in Ramadan. Allah u says:

﴾فَمَن شَهِدَ مِنْكُمُ ٱلشَّهْرَ فَلْيَصُمْهُ﴿

Therefore, he who witnesses this month must fast it.

It is definite that spiritual retreat founded on the Prophetic practice cannot be more than ten days and the days of Ramadan cannot exceed 30. So, he must start his spiritual retreat on the 18th of Ramadan in Pakistan, which would be his 20th day of fasting, based on the date he started it. He will complete his spiritual retreat on the 28th of Ramadan, for Ramadan cannot have more than 30 days. He will not fast the next day, the 29th in Pakistan but can observe optional spiritual retreat if he wants to.

'Abd Allah bin 'Umar (R.A) says that the Prophet a said as he mentioned Ramadan.

لَا تَصُوْمُوا حَتَّى تَرَوُا الْهِلاَلَ، وَلاَ تُفْطِرُوا حَتَّى تَرَوْهُ، فَإِنْ أُغْمِيَ عَلَيْكُمْ فَاقْدِرُوْا لَهُ.

‘Do not fast until you sight the crescent and do not break fast until you sight it. If it is cloudy for you then count it fully (make it the month of 30 days).’

So, it is important to start the month where you have sighted the moon and complete 30 days when you are in a different country, where the moon is not visible, when you complete your 29th fast, for no lunar month can have more than 30 days.